In an electronic power steering apparatus, an electric machine (motor) generates a steering assisting force corresponding to the magnitude of a steering torque, and this steering assisting force is transmitted to a steering system so as to reduce a steering force in steering by a driver. Disclosed is a technology for controlling an electric machine, wherein a base current (assist torque) defined by a steering torque and a vehicle speed is compensated by the inertial of a steering system and adjusted by the damper (viscosity) of the steering system, and the current compensated by the inertial and adjusted by the damper is taken as a target current (see Patent Documents 1 and 2).
Incidentally, only torque sensor output is input to an inertial compensating current value determining means disclosed in Patent Document 2, and a vehicle speed signal is not input.
In Patent Document 3, disclosed is a technology, wherein, in order to solve the problem that a self-aligning torque applied to the steering wheel by wheels and a steering mechanism is small and the returning force of the steering wheel is weak and control of the steering wheel by the driver tends to be destabilized by irregularity, rolling, or the like of a road surface, as a solution, an electronic power steering apparatus controls such as to generate a steering assisting force, based on the deviation between a preset target steering torque for a steering angle detected by a steering angle detection sensor and an actual steering torque detected by steering torque sensor.
Further, it is shown in FIG. 8 of Patent Document 4 that, regarding steering angle vs. steering torque characteristic of a vehicle adopting a steering geometry with characteristics similar to those of a parallel geometry, when the steering angle is small, the steering torque becomes larger as the steering angle increases, and the steering torque becomes smaller as the steering angle subsequently becomes larger. This is a factor that gives a feeling of strangeness to a driver. FIG. 9 of Patent Document 4 shows steering angle vs. steering torque characteristic with an ideal Ackermann geometry, wherein the steering torque increases linearly in proportion to the steering angle.
With a vehicle designed with a parallel geometry in such a manner, as the transverse sliding angle of a turning inner wheel turns to the opposite direction, when the self-aligning torque turns to the divergence side and the steering angle becomes large, the steering torque becomes small, which gives a driver a feeling of strangeness such that control of the steering wheel by the driver is destabilized.
As a solution in this situation, Patent Document 4 discloses a technology for adjusting steering feeling corresponding to the steering angle by a steering wheel returning control function, wherein a control device of an electronic power steering apparatus includes a steeling angle sensor for detecting a steering angle, a steering torque sensor for detecting a steering torque, a vehicle speed detecting means for detecting a vehicle speed, and a control means for controlling an electric machine, based on the steering angle, the steering angular speed of the steering angle, the steering torque, and the vehicle speed, and wherein the control means has the steering wheel returning control function to control returning of the steering wheel.